Erbium doped-fiber amplifier (EDFA) systems are well known as useful for amplifying an electromagnetic radiation signal, for example a photonic optical signal. In such amplifier system configurations, input and output waveguides for carrying such an optical signal are interposed by gain media constituted by an inorganic glass amplifier waveguide fiber which is doped with a lanthanide element. The lanthanides have incomplete inner electron orbitals that facilitate excitation of electrons into higher energy levels by applied photonic energy referred to as pumping radiation. When the optical signal is carried into the amplifier waveguide fiber and itself applies further photonic energy to such lanthanides, the excited electrons are triggered to return to lower energy states and to simultaneously release photons. The release of these photons is referred to as fluorescence. Repeated excitation and fluorescent emission takes place as a chain reaction within the amplifier waveguide fiber, modulated by the optical signal. As a result, the output waveguide carries an amplified copy of the optical signal received from the input waveguide. Luminescence generally cannot be generated by direct excitation of lanthanide atoms due to their poor ability to absorb light. Organic chromophores can be provided to absorb the light energy and transfer it to nearby lanthanide atoms.
Optical waveguide fibers typically are fabricated from conventional inorganic glass materials such as silicon dioxide or silica, including dopants that generate an appropriate refractive index profile and otherwise enable the optical waveguide fibers to carry an optical signal. Inorganic glass is particularly the material of choice for fabricating optical fibers for long haul optical telecommunications systems. Inorganic silica glass materials, for example, have excellent thermal stability, and on a molecular scale are essentially free of light scattering and phase separation. A given optical fiber incorporated in a long haul system can traverse distances measured in miles without requiring any interconnection splicing of fibers. However, interconnection splicing of inorganic glass optical fibers, when ultimately required, is a delicate operation. The core of a typical inorganic glass optical fiber is very small, making the mutual lateral alignment of optical fiber ends a tedious process requiring specialized connecting devices and procedures. Connections are typically made by installing optical fiber connectors on the fiber ends, and then assembling the connectors together.
In short haul applications, optical waveguide fiber lengths between connection points may be short, due to complex mesh network interconnections of optical waveguide fibers, together with the needs for a variety of interposed components such as EDFAs, add drop multiplexers, optical cross connects, and other components. Hence, required optical waveguide fiber interconnections in short haul applications, such as office local area networks (LANs) and intravehicle mobile communication systems, may proliferate to extremely large magnitudes.
In addition to the labor and connector device costs inherent in short haul applications, the cumulative weight of the resulting network nodes can be detrimental in certain applications. For example, in an aircraft equipped with complex networked telecommunications systems, the cumulative weight of the inorganic glass fibers embodied in these systems can be detrimental to the aircraft performance.
Organic plastic optical waveguide fibers are an available alternative to inorganic glass optical waveguide fibers. Organic plastic materials typically have a substantially lower density than does, for example, silicon dioxide. Organic plastic optical waveguide fibers, however, are typically characterized by higher levels of signal attenuation, and inferior mechanical stability, compared with inorganic glass optical waveguide fibers. Organic plastic materials are subject, for example, to creep and other losses of mechanical integrity. They also typically contain compositional impurities and phase separated imperfections that can cause light scattering and attenuation. Organic plastic planar waveguides, to which the same comments apply, are an analogous type of optical transmission architecture. However, organic plastic planar waveguides typically are utilized in optical integrated circuit applications.
As explained above, interconnection splices between inorganic glass optical fibers require careful lateral alignment and specially adapted connectors. In addition to mutual lateral alignment between spliced fibers, the spliced fiber ends must also be aligned at a tolerable distance from each other in order to carry a signal. Fiber ends too close together can lead to impacting of the ends, resulting in fiber misalignment and signal failure, and fiber ends too far apart can also lead to signal failure. Diverse materials have correspondingly diverse coefficients of thermal expansion, typically with adverse consequences from environmental temperature excursions. Accordingly, amplifier waveguide fibers for EDFAs to be used together with inorganic glass optical fibers typically are likewise made from inorganic glass materials to facilitate their interconnection and integration with conventional optical waveguide fiber network systems. However, amplifier waveguide fibers made from inorganic glass materials are not well suited to make EDFAs to be interconnected with organic plastic optical waveguide fibers, for example due to the mismatch in thermal expansion coefficients.
Inorganic glass and organic plastic precursor materials can be combined together by sol-gel technology for use in making polymeric optical fiber waveguides, both for use as signal transmission carriers in long- and short-haul telecommunications and as doped with lanthanides to be employed as gain media in EDFAs and other active devices. The resulting organic/inorganic glass hybrid materials offer properties that are not available from organic or inorganic materials alone. Their coefficients of expansion can be made compatible with those of conventional organic plastic materials. Thus, sol-gel technology constitutes a potential solution to the need for organic plastic optical waveguide fibers that effectively resolve the telecommunication fiber weight problem, and the need for mutually compatible gain media for active devices such as EDFAs. Upconversion lasers are an exemplary further class of active devices in which the active materials can be produced by combining together inorganic glass and organic plastic precursor materials by sol-gel technology.
Sol-gels are produced by condensation of reagents containing —Si—OH groups, with release of water as a byproduct. These reagents can contain any desired organic groups. Since the reactions can be carried out at room temperature, the organic groups can be conserved in the polymerized system. However, a high level of uncondensed hydroxyl groups generally remains in materials prepared by the sol-gel process. Such hydroxyl groups reduce the fluorescent efficiency and shorten the luminescent lifetimes of lanthanide dopant ions in the material, adversely affecting optical signal amplification performance. In addition, there is a tendency for lanthanide ion clustering to occur in sol-gel systems. Ion clustering of lanthanides leads to self quenching of fluorescent emissions between lanthanide atoms that are too close together, reducing the total fluorescence.
Various sol-gel polysesquioxane compositions have been produced, including those doped with lanthanides. However, there is a need for improved polysesquioxane hybrid polymer compositions that are engineered for use as gain media in EDFAs, as active elements in upconversion lasers, and in other active applications. Such improved active hybrid polymeric systems would enable fluorescence at power levels suitable for signal amplification purposes, and address the problem of lanthanide ion clustering. Such active doped hybrid polymer material based systems would also tolerate interconnection of active elements with organic plastic-based optical fibers, facilitating the assembly of organic plastic fiber based telecommunications components, networks and systems. There is also a need for processes for making such polysesquioxane compositions, and for lanthanide doped media fabricated from such compositions that can be used in active devices such as optical fiber amplifiers, upconversion lasers, and other devices requiring elements for generation of lanthanide pumping fluorescent emissions.